feat(ui): sidebar lists workspaces with their sessions, Claude Desktop style

The sidebar's project tree becomes a flat workspace list (WorkspaceTree):
one row per workspace in the stored order, each expanding to its sessions.
A session that still has a live pane opens /workspace?lane=<id>&run=<runId>,
which only attaches pane 0 to that tmux session; any other session opens
its read-only transcript. Nothing in the sidebar starts or resumes a run.

client/src/components/run/__tests__/LaneConsolePane.test.tsx

@@ -526,4 +526,34 @@ describe("LaneConsolePane", () => {
     await screen.findByTestId("terminal-view");
     expect(api.lanes.action).toHaveBeenCalledTimes(1);
   });
+
+  it("attaches to a run picked in the sidebar without starting or resuming anything", async () => {
+    vi.mocked(api.run.get).mockResolvedValue({
+      id: "agenthub-lane-1-2",
+      laneId: 1,
+      slot: 2,
+      status: "running",
+      cwd: "/workspace/a",
+      model: null,
+      permissionMode: null,
+      effort: null,
+      resumeSessionId: null,
+      sessionId: "s-9",
+      startedAt: null,
+      promptPreview: null,
+      tool: "claude",
+    });
+    render(
+      <MemoryRouter>
+        <LaneConsolePane
+          {...baseProps()}
+          attachRequest={{ runId: "agenthub-lane-1-2", nonce: "k1" }}
+        />
+      </MemoryRouter>
+    );
+    await screen.findByTestId("terminal-view");
+    expect(api.run.get).toHaveBeenCalledWith("agenthub-lane-1-2");
+    expect(api.lanes.action).not.toHaveBeenCalled();
+    expect(api.run.start).not.toHaveBeenCalled();
+  });
 });